The disclosed invention generally relates to transverse discharge excitation lasers, and is particularly directed to an electrode structure for transverse discharge excitation lasers which minimizes extraneous electrode or feed related discharges.
In a transverse discharge excitation laser, an RF excitation electric field is applied transverse to the longitudinal dimension of a laser excitation cavity. An example of a transverse discharge excitation laser is set forth in U.S. Pat. No. 4,169,251, issued to Laakmann on Sept. 25, 1979, and assigned to the assignee of the present invention. In the system of U.S. Pat. No. 4,169,251, the electrodes form two opposing walls of the laser cavity. Such electrodes are internal electrodes which are in contact with the laser gas subject to the RF excitation field.
Further examples of transverse discharge excitation lasers are set forth in U.S. application Ser. No. 745,570, filed on June 17, 1985, by P. F. Robusto, and assigned to the assignee of the subject invention. Included therein are examples of laser structures wherein the electrodes are dielectrically isolated from the laser cavity. Such electrodes are external to the laser cavity and are not in contact with the laser gas subject to the RF excitation field.
In operation, the laser gas should be in contact with a larger gas reservoir to provide for a longer laser lifetime. The foregoing can be accomplished by one of several techniques. With one technique, the laser cavity is hermetically sealed as a vacuum vessel and is in communication with a separate gas ballast volume. With another technique, the laser structure is contained within a vacuum vessel which is filled with the laser gas.
Considerations involved in the use of a hermetically sealed laser cavity include the complexity of achieving a proper seal, which may be particularly difficult for long laser structures and for folded laser structures. Another consideration with the use of a hermetically sealed laser cavity is the connection between the laser cavity and the separate gas volume, since such connection tends to be fragile. Further, the hermetic seals also tend to be fragile, and as a result of difficult and complex manufacturing requirements cause low manufacturing yield.
Considerations involved in the use of a vacuum vessel-contained RF laser structure include extraneous RF discharges outside the laser cavity. Typically, the vacuum vessel is made of a conductive metallic material for strength, ease of manufacture, and use as an RF shield. An important cause of extraneous RF discharges is the capacitive coupling between the conductive vacuum vessel and non-grounded electrode(s) that is created by the dielectric structure of the laser. Sharp edges in an electrode produce localized electric fields of sufficient intensity to cause discharges of the non-active laser gas outside the laser cavity.
It should be noted that techniques for suppressing extraneous discharges in DC lasers are generally inapplicable to RF lasers. For example, in DC lasers ceramic may be utilized as insulation for suppressing extraneous discharges. Such use of ceramic in RF lasers would provide increased capacitive coupling that would tend to promote extraneous discharges.
A known approach to avoiding extraneous discharges in vacuum vessel-contained RF laser structures is the use of bulk internal electrodes which cooperate with a ceramic structure to define the laser cavity. However, the different thermal coefficients of the electrodes and the ceramic are an important consideration with respect to achieving and maintaining proper alignment.
Extraneous RF discharges may also be avoided by providing sufficient separation between the non-grounded electrode(s) and the vacuum vessel to eliminate extraneous RF discharges. However, that would result in larger and more costly lasers.
A further technique for preventing extraneous RF discharges in vacuum vessel-contained RF laser structures is the use of gas pressures and/or excitation frequencies which are higher than optimum. However, higher pressures require higher operating voltages and reduce laser operating efficiency. Higher operating frequencies may reduce laser operating efficiency, and may exceed the FCC specified operating frequency, which would either require obtaining an FCC exemption or preclude commercial applications.